Queensland University of Technology A study into the permeability and compressibility properties of Australian bagasse pulp By Thomas J. Rainey B.Eng (Chem), Hons I A THESIS SUBMITTED FOR THE DEGREE OF DOCTOR OF PHILOSOPHY School of Engineering Systems Queensland University of Technology 2009
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Queensland University of Technology
A study into the permeability and
compressibility properties of
Australian bagasse pulp
By
Thomas J. Rainey
B.Eng (Chem), Hons I
A THESIS SUBMITTED FOR THE DEGREE OF
DOCTOR OF PHILOSOPHY
School of Engineering Systems
Queensland University of Technology
2009
i
IMPORTANT NOTICE
The information in this thesis is confidential and should not be disclosed for
any reason nor relied on for a particular use or application. Any invention or
other intellectual property described in this document remains the property of
Queensland University of Technology.
Thomas J. Rainey, A study of bagasse pulp filtration
Determination of the Permeability Parameters of Bagasse Pulp from Two Different Sugar Extraction Methods. In Proceedings Tappi Engineering Pulping and Environmental Conference, Session 4.1, Portland, Oregon, USA. (2008).
Rainey, T., Brown, R., Martinez, D.M., and Doherty, B. - The use of CFD to
simulate the behaviour of bagasse pulp suspensions during the dewatering process, Appita conference, Melbourne. (2006). Available online at QUT e-prints.
Several of the above papers are available online at the following site: http://eprints.qut.edu.au/
Thomas J. Rainey – A study of bagasse pulp filtration
ix
Acknowledgements
I would like to thank all of my supervisors, without whom this thesis
would not be possible. I would like to thank Bill Doherty for helping me cope
with much of the structure and direction of my work – your door was always
open to me; Richard Brown - you always helped me with resource issues and
smooth progression through the milestones; Mark Martinez at UBC for
assistance with developing the dynamic filtration model and the long
discussions interpreting my data; and Neil Kelson for helping me with the
coding and article writing. All of you helped me in many ways.
This work would not have been possible without the financial
contribution of the Federal Government’s Sugar Research and Development
Corporation PhD scholarship fund, and income support from QUT’s Sugar
Research and Innovation. I would like to thank the Queensland Government’s
financial contribution through the PhD Smart State Fund. I also acknowledge
the financial contribution of the Faculty of Built Environment and Engineering
and the Engineering Systems theme. Your significant financial contributions
are deeply appreciated.
I would like to thank my family, particularly Jenni for her unending
patience through all those sleepless nights. Thank you Mum and Dad for your
support. Thank you Anna for entertaining me through my final year.
I would like to acknowledge the generous in-kind contribution of the
following organisations: the Australian Pulp and Paper Institute, particularly
Loi Nguyen, for use of their facilities; CSR Sugar and Mr. Paul Turnbull for
assistance with collection of the bagasse; Covey Consulting (Geoff Covey);
1.1.1. The production of sugar and bagasse from sugarcane 3 1.1.2. Potential uses of bagasse 6 1.1.3. Paper manufacture 7 1.1.4. Issues with bagasse paper manufacture in the
Australian context 10 1.1.5. The benefits of flocculants to assist paper formation 12
1.2. Research aim 12
1.3. Statement of objectives 12
1.4. Statement of novelty 14
1.5. Summary of thesis chapters 16
Chapter 2 Theory and Literature Review -------------------- 19
2.1. Background 19
2.2. Bagasse pulp properties 21
2.2.1. Bagasse pulp yield 21 2.2.2. Bagasse pulp fibre morphology 22 2.2.3. Chemical character of bagasse pulp fibres 23 2.2.4. Bagasse pulp physical properties 24
2.3. Pulp permeability and compressibility parameters 26
2.3.1. Steady-state permeability theory 26 2.3.2. Steady-state compressibility theory 35 2.3.3. Dynamic filtration theory 36 2.3.4. Non-Darcy flow 39 2.3.5. Equipment used in filtration studies 40 2.3.6. Additional filtration theory of particular importance to
this study 48
2.4. Chemical additives 52
2.4.1. The mechanism of CPAM and microparticle dual polymer systems for pulp flocculation 52
2.4.2. Flocculant systems 53 2.4.3. Literature on flocculants used for bagasse pulp 55
Thomas J. Rainey, A study of bagasse pulp filtration
xiv
2.4.4. Using the Dynamic Drainage Jar as a tool for comparing flocculants 56
2.4.5. Summary of chemical additives literature and theory 57
2.5. Summary of theory and literature review 58
Chapter 3 Experimental procedure and modelling----------60
3.1. Overview of experimental and modelling methodology 61
3.1.1. Preparation of Australian bagasse pulp 62 3.1.2. Physical and chemical property testing 63 3.1.3. Steady-state permeability property testing 63 3.1.4. Steady-state compression testing 64 3.1.5. Dynamic filtration modelling and verification 64 3.1.6. Effect of chemical additives on the drainage and
retention properties 65 3.1.7. Flow diagram of the experimental and modelling
methodology 65
3.2. Bagasse pulp preparation 67
3.2.1. Collection of raw materials 67 3.2.2. Pulp sample preparation 71 3.2.3. Test for statistical significance between two
populations of pulp samples 76
3.3. Physical and chemical property testing procedure 78
3.3.1. Chemical characterisation of pulp and bagasse 78 3.3.2. Pulp physical property testing 79 3.3.3. Fibre length analysis 80 3.3.4. Microscopy investigation 81
3.4. Steady-state permeability testing equipment and experimental procedure 83
3.5. Quasi steady-state compressibility experimental procedure 88
3.6. Dynamic filtration modelling and experimental verification procedure 90
3.6.1. Dynamic filtration modelling procedure 91 3.6.2. Verification of the dynamic model 91
3.7. Equipment and procedure for testing the effect of chemical additives 92
3.7.1. Methodology – Effect of shear 94 3.7.2. Methodology – Effect of vacuum 95 3.7.3. Methodology – Effect of chemical additives on
permeability and compressibility. 97
3.8. Summary of the experimental investigation 97
Chapter 4 Results and discussion -------------------------------99
Thomas J. Rainey – A study of bagasse pulp filtration
xv
4.1. Results of bagasse chemical pulping 100
4.1.1. Bagasse pulping kinetics 100 4.1.2. Effect of bagasse pre-treatment on Australian bagasse
pulp yield 101 4.1.3. Effect of bagasse pre-treatment on bagasse pulp kappa
number 103 4.1.4. Summary of bagasse pulping analyses 103
4.2. Results of physical and chemical property testing 104
4.2.1. Pulp chemical analysis results 104 4.2.2. Pulp physical property results 107 4.2.3. Fibre length distribution analysis 112 4.2.4. Microscopic analysis 115 4.2.5. Summary of pulp physical and chemical property
testing 116
4.3. Results of steady-state permeability testing 117
4.3.1. Data from steady-state permeability testing 117 4.3.2. Effect of bagasse pre-treatment on pulp permeability
properties 121 4.3.3. Review of bagasse pulp steady-state permeability
model 122 4.3.4. Comparison of steady-state permeability data with
previous work 123 4.3.5. Summary of steady-state permeability experiments 125
4.4. Results of quasi steady-state compressibility testing 126
4.4.1. Suitability of the power law steady-state compressibility model 127
4.4.2. Pulp steady-state compressibility data and comparison with the findings of previous workers 128
4.4.3. The effect of pre-treament on bagasse compressibility 129 4.4.4. Summary of steady-state compressibility testing 131
4.5. Results of dynamic filtration modelling and validation 131
4.5.1. Predictions of the dynamic model 131 4.5.2. Dynamic filtration experiments and comparison with
predicted values 136 4.5.3. Summary of dynamic filtration modelling 137
4.6. Results of chemical additives testing 139
4.6.1. The effect of shear and additives on pulp retention 140 4.6.2. The effect of chemical additives and vacuum 146 4.6.3. The effects of chemical additives on permeability and
compressibility parameters 151 4.6.4. The effect of chemical additives on bagasse pulp’s
dynamic filtration behaviour 158
Thomas J. Rainey, A study of bagasse pulp filtration
xvi
4.6.5. Summary of the effect of chemical additives on pulp permeability and compressibility 161
Figure 2.4 Schematic of a 1-D flow cell where a piston is expressing
liquid through an initially un-networked suspension.
This thesis considers the permeability and compressibility of a pulp pad
using an initially networked model.
2.3.4. Non-Darcy flow
The above equations all assume laminar flow. For flow at a sufficiently
high velocity, turbulence occurs and the more complex Forcheimer equation
can be used instead of Darcy’s Law (103).
2
vv*K
P βρ+µ
=∇−
Equation 2.22
Where � is an experimental constant and K* is a permeability constant
that is analogous to that used in Darcy’s Law.
Thomas J. Rainey, A study of bagasse pulp filtration
40
2.3.5. Equipment used in filtration studies
This thesis uses an initially networked model for the permeability and
compressibility study since the steady-state permeability and compressibility
parameters. These parameters can be measured using simple equipment (see
sections 2.3.5.1). However, optimisation of a good chemical additives system
must be performed using equipment where a slurry is used rather than a pulp
pad, i.e. the pulp is initially un-networked. The equipment used for these
studies is described in section 2.3.5.2.
2.3.5.1.Equipment used for permeability and compressibility testing of pulp pads
The equipment used by various workers into the permeability and
compressibility testing of pulp pads are fairly simple (81-86). Sometimes, the
focus of the investigation is on pad permeability only (86). Some
investigations attempt to measure the compressibility and permeability
properties simultaneously (81-85). The equipment in all of these investigations
involves loading a pulp slurry into a (most commonly) transparent vessel. For
permeability studies, the pressure gradient is measured by manometers as well
as the water flow rate. For compressibility studies, the response of a hydraulic
or mechanical load to the height of the pulp pad is measured.
Figure 2.5 shows the cell used by Robertson and Mason which is fairly
typical of permeability and compressibility studies. In this particular
arrangement, the pulp is loaded into a 40 mm cylinder which is bounded by a
plunger with a reinforced 100 mesh screen at the top of the pad. The flow rate
is measured by timing the drop in level of a measuring cylinder, and the
pressure head is measured by the difference in fluid height between the cylinder
and a side arm.
Chapter 2 - Theory and Literature Review
41
Figure 2.5 Sketch of a permeability cell used by Robertson and Mason
(86).
For the case of measuring compressibility simultaneously, the plunger
compresses the pulp pad in the experiment and there is continuous monitoring
of the flow rate and pressure.
In previous permeability and compressibility studies, the temperature of
the water was not reported, so it is assumed the experiments were carried out at
ambient temperature. The author acknowledges that industrial paper forming
occurs at elevated temperature which affects pulp pad permeability and
compressibility. Ambient temperature was used in this study in order to be
consistent with that used by previous workers for comparative purposes.
In reality, paper manufacture involves thin pulp mats rather than thick
pulp pads. Experimentally measuring the permeability and compressibility of
thin pulp mats is difficult, and beyond the scope of this study. For this reason,
previous authors, as well as this author, measured the permeability and
compressibility of pulp pads because it only requires very simple equipment. It
is common practice to use the results from experiments with pulp pads to
represent the behaviour of pulp mats.
Thomas J. Rainey, A study of bagasse pulp filtration
42
2.3.5.2.Equipment for filtration studies of fibre suspensions
A review of equipment used previously for accurately simulating sheet
formation on a paper machine, from pulp slurry, was conducted. For this study,
equipment that was well suited to testing the effects of chemical additives under
shear conditions and vacuum was investigated.
A Dynamic Drainage Jar (DDJ) is a simple stirred vessel into which a
dilute pulp slurry and flocculants are added. The water and fine fibrous
material passes through a permeable screen with 75 �m holes and the fraction
of fines retained above the screen is measured. By increasing the stirrer speed
(i.e. shear), the effectiveness of flocculants under high shear conditions can be
determined. Flocculation effectiveness is measured by the retention of fine
fibrous material. A higher level of retained fines over a wide range of stirrer
speeds means that flocculation has improved. The DDJ is most commonly used
to test the effectiveness of various flocculants and so it is discussed in more
detail in section 2.4. A DDJ was used in this study.
Figure 2.6 Sketch of a Dynamic Drainage Jar.
Chapter 2 - Theory and Literature Review
43
Many modifications of the DDJ exist for simulating the industrial
forming process. Modifications of the DDJ are the subject of a number of
papers (for example 104, 105). Modifications of the DDJ usually permit pad
formation to look at the combined effect of mechanical entrapment and
colloidal interaction.
The DDJ was modified by Britt and Unbehend in 1980 (106) to measure
the dryness of a sheet after exposure to a controlled vacuum. The vacuum was
applied to simulate the suction created on the Fourdrinier former; however, it
was not a pulsed vacuum. Britt and Unbehend (106) describe a method for
testing a dynamic drainage rate. An observation was that over-flocculation
created channels in the fibre pad, improving the initial drainage rate but once
the water was removed from the interstices of the pad air was sucked through
the sheet when vacuum was applied, resulting in higher final sheet moisture
content. Pulp suspensions of lower initial drainage rate tended to form more
consolidated and uniform sheets which when subjected to vacuum resulted in a
sheet of lower moisture content.
In 1982, the application of vacuum to a modified DDJ was automated
(107). In trials, Britt applied vacuum for 5 s and measured the final consistency
of the pad. In 1985, Britt further illustrated that pulp which drained quickly had
poor final dryness when vacuum was applied and that a certain level of fines
can improve the final sheet dryness (108). The following mechanism was
suggested: when the shortest fibres are mobile with respect to the fibre pad, the
fines migrate to the interstices of the forming web, sealing or plugging some of
the openings and slowing drainage but when the fines are flocculated and
attached to fibres, they are no longer free to migrate to the interstices and
drainage is not impeded.
The modification by Forsberg, called the “Dynamic Drainage Analyser”
(DDA) in (104) involved a microprocessor which served two functions; to
control (i) the duration of chemical addition in order to investigate contact time
between retention chemicals and fibres and (ii) the duration of stirring in order
to investigate different shear conditions. In this arrangement, it was intended to
form a pad to investigate mechanical entrapment of fibres as well as the
Thomas J. Rainey, A study of bagasse pulp filtration
44
colloidal interaction. See Figure 2.7, for an illustration of the arrangement.
The DDA provided information on retention, drainage, porosity and wet web
dryness. The DDA recorded the vacuum level as a function of time. Figure 2.8
shows that the graphical output provided information on the drainage rate under
vacuum (time from point a to point c) and the final porosity of the dry sheet
(magnitude of vacuum at point d). The modified DDJ used in this study most
closely resembles Forsberg’s DDA.
Figure 2.7 Diagram of Forsberg’s Dynamic Drainage Analyser (104).
Figure 2.8 Graphical output of the DDA(104).
Although laboratory equipment that more accurately simulates the
industrial forming process exists, they came with increasing cost. In order to
Chapter 2 - Theory and Literature Review
45
improve the behaviour of bagasse pulp, a reasonably accurate and affordable
method of simulating the pulp dewatering process must be achieved in the
laboratory.
In order to further improve the similarity between laboratory equipment
and a Fourdrinier paper former, it was necessary to introduce pressure pulses
into the equipment. Various modifications of the DDJ attempted to incorporate
pulsed vacuum such as that described by Hubbe (105).
The modification by Hubbe, the “Positive Pulse Jar” (PPJ) (105) as
shown in Figure 2.9, introduced pressure pulses by a Bellows pump, pumping
dilutant under the jar. Previous versions had introduced pulses by vacuum
pump. The advantage of this method was that it more accurately simulated the
refluidising of the fibre mat facing the fabric, resulting in reduced fines content
in this region. The PPJ also investigated the use of a specialised rotor to
simulate uniform shear, as opposed to the random turbulence obtained in a
standard DDJ. The pressure pulses reduced retention. Importantly, the use of
the specialised rotor also resulted in reduced retention compared to the standard
impeller used by Britt.
Figure 2.9 Diagram of the PPJ and specialised rotor (105).
The Australian Pulp and Paper Institute (APPI) have pilot laboratory
equipment that more accurately simulates a Fourdrinier style paper machine;
see Figure 2.10 taken from Xu and Parker (109). It contains a moving belt with
hydrofoils attached in order to simulate the pressure pulses of a Fourdrinier
former. The equipment does not take into account the velocity profile of the
stock leaving the headbox slice.
Thomas J. Rainey, A study of bagasse pulp filtration
46
Figure 2.10 Moving Belt Drainage Former (109).
Melbourne University in conjunction with CSIRO Forestry and Forest
Products (now called Ensis) also developed laboratory forming equipment that
simulates the velocity profile of the stock leaving the slice of an industrial
paper machine (110-112). This configuration is shown in Figure 2.11.
Importantly, this equipment is capable of aligning the fibres, approximating
fibre alignment on a paper machine. It does not take into account pressure
pulses characteristic of a Fourdrinier former.
Figure 2.11 Setup of the laboratory former by Helmer (110-112).
Another sophisticated piece of laboratory equipment outlined by Kataja
and Hirsila (113) should be mentioned here. Although it does not simulate the
forming process as accurately as the laboratory formers, it can be used to obtain
very detailed data about pulp pad formation; more than any other laboratory
equipment encountered. The equipment used by Kataja and Hirsila can be used
to measure the velocity of fibres at various heights in a dewatering fluid flow,
Chapter 2 - Theory and Literature Review
47
see Figure 2.12. This equipment is particularly useful for developing numerical
models of pulp suspension behaviour. The unit consists of a sealed tank with a
riser tube. Inside the riser tube is a wire and support grid. The suspension of
fibre is allowed to drain through the wire. The fibre is retained on the wire and
forms a fibre mat. The water level inside the riser is measured with an
ultrasonic surface detector. The vertical velocity of the fibres is measured
through the wire by four pulsed ultrasound Doppler anemometers. The signal
sent from the surface detector and the anemometers are processed and the data
is captured. The probes can measure the vertical velocity of the fibres up to
70 mm above the wire. The water level in the riser tube is adjusted by valves
V1, V2 and V3. Assuming valves V1 and V3 have adequate accuracy and
response time, the programmable logic could be modified to allow a time-
varying pressure pulse, simulating the effect of foils in Fourdrinier forming.
El-Sharkawy (50) used the equipment outlined by Kataja and Hirsila to
control bagasse pulp quality through fractionation and refining (50). This work
is the only published work with data presented on the drainage properties of
bagasse pulp.
Figure 2.12 Ultrasound anemometry for measuring filtration of fibre
suspension (113).
Thomas J. Rainey, A study of bagasse pulp filtration
48
To measure the efficacy of chemical additives under both vacuum and
shear, a modified DDJ that most closely resembles that developed by Forsberg
was used for this project. It was simple to construct and gives an indication of
how chemical additives would perform under the dual effects of shear and
vacuum. The main differences are that it will involve a laptop computer to log
the data, the vacuum will be controlled by actuating a bleed valve on the
vacuum vessel and flow rate will be measured with digital scales. The
modified DDJ more closely resembles a Fourdrinier former than a Twin-wire
former.
2.3.6. Additional filtration theory of particular importance to this study
2.3.6.1.Steady state permeability theory
In the case of pulp fibres in the swollen state, a considerable amount of
water occupies the pores of the fibres. Incorporating � into the Kozeny-Carman
model (Equation 2.11) allows this study to obtain information on potential
strength generation during refining as well as permeability data. If the swelling
factor of the fibres is � cm3/g, then the porosity is related to the concentration, c
g/cm3, by � = 1 – �c.
Inserting into (Equation 2.11) and rearranging obtains
( ) ( )c1Sk
1Kc
3/1
2
v
2
3/12 α−���
����
�
α=
Equation 2.23
Plotting (Kc2)1/3 against concentration, c, will give a linear relation.
Darcy’s permeability, K, is determined from permeability experiments using
equation (1) and c is calculated from the height and diameter of the pulp pad for
a known mass of pulp. The specific surface area, Sv, and the swelling factor, �,
are calculated from the slope and the intercept of the graph. This method was
first used by Robertson and Mason (86). Sv and � are then inserted back into
equation (2) to test the agreement of the experimental data with the Kozeny-
Carman model. The Kozeny factor, k, is frequently assumed to be constant.
For randomly packed fibrous beds, k was determined to be 5.55 (114).
Chapter 2 - Theory and Literature Review
49
Values for � are reported in the same literature in which Sv is reported
(81-86). Reported values of � vary more than the reported values of Sv.
Ingmanson and co-workers report values as low as 1.65 cm3/g for wood pulp
whilst Robertson and Mason report � as high as 4.5 cm3/g for a sample of never
dried wood pulp.
The advantage of this method for quantifying the steady-state
permeability of pulp samples is that it requires a very simple experimental
method and also the equipment required is extremely simple. A pulp pad is
created by draining pulp slurry into a transparent vessel which is reinforced at
the bottom by a mesh. The flow rate is measured using a collection vessel and
stopwatch. The pressure drop per unit length can be measured by manometers.
The height of the pulp pad is measured to determine the pulp concentration,
assuming that the pulp concentration is approximately uniform in the absence
of significant hydraulic pressure. Darcy’s permeability constant, K, can be
determined from this data.
2.3.6.2.Steady-state compressibility theory
The compressibility equipment was designed so that the pulp could be
loaded into a cell and compressed with a permeable top platen which expresses
water. This is shown diagrammatically in Figure 2.13, indicating the pressure
on the solid phase, Ps, at the platen. The distance, x, is defined from the top
platen.
The hydraulic pressure at the top surface of the pulp pad is negligible so
the force on the fibres equals the force exerted on the platen.
The compression model used is the power law model viz Ps = M c N
Thomas J. Rainey, A study of bagasse pulp filtration
50
Figure 2.13 Sketch of the compressibility cell.
The pulp concentration can be related to the solidity (that is, the volume
solids fraction), which is used in the dynamic model, by � = �c. Values for �
are in the range of 3.2-3.8 cm3/g.
For this geometry,
Ps=m �n
Equation 2.24
where m and n are experimental constants analogous to, and calculated
from, M and N.
2.3.6.3.The dynamic filtration model
This study follows the analysis of Landman and co-workers (100) for a
one dimensional constant rate filtration (i.e. platen moves with constant speed)
using an initially networked suspension. Martinez has built on this work for
pulp and paper applications (115, 116). The modifications include the
incorporation of the Kozeny-Carman steady-state permeability model (Equation
2.11) and the power law steady-state compressibility model (Equation 2.24).
The derivation of the governing equations for the filtration of a pulp pad is
provided in Appendix A.
Using the definitions presented in Figure 2.13, the dimensional form of
the governing equation for constant rate filtration is
Expressed
water
x
Depth into the pulp
mat
Ps
Height, h
Applied pressure
Loaded
pulp
Permeable
top platen
Impermeable
base
Chapter 2 - Theory and Literature Review
51
( )dx
d
dt
dh
dx
dD
dx
d
dt
d φ−��
�
� φφ=
φ
Equation 2.25
Where
( ) ( ) ( )�
mnK 1D
1n−φφφ−φ=φ
Equation 2.26
K(�) is the permeability as predicted by the Kozeny Carman model
(Equation 2.11). This governing equation is subjected to the initial condition
�(x,0) = �0 as the solidity is uniform throughout the cell, as well as the
following boundary conditions:
Boundary condition at the top platen
dt
dhu 0,x −==
0dx
d=
φ
Equation 2.27
Boundary condition at the base
0u 0,x ==
( )( ) 1nmn1Kdt
dh
dx
d−φφ−φ
µ=
φ
Equation 2.28
Solution of the dynamic model requires the factors m and n calculated
from steady state compression experiments, Sv and � from permeability
experiments. These equations are non-dimensionalised before being solved
(see Appendix A for the non-dimensional equations). The solution of these
equations provides values for � over the ranges of x and t.
For comparative purposes, the model predictions for � are determined at
x=0 for all t and consequently Ps is calculated (Equation 2.24). In the
experimental setup, the Instron measures the load on the top platen which is
converted to pressure. Validation of the model occurs if the experimental
pressure data matches the model predictions for solids pressure at the surface of
the pulp pad.
Thomas J. Rainey, A study of bagasse pulp filtration
52
Both a constant k (k = 5.55), and variable k (Equation 2.12) with the
relevant values of Sv and �, are investigated for use in the dynamic model.
The dynamic model assumes that 100% of the fibre is retained by the
platen and also neglects friction between the platen and the side wall.
The effect of applying vacuum at the bottom boundary, which occurs in a
Fourdrinier former has the same effect as increasing the pressure at the top
boundary.
2.4. Chemical additives
As mentioned in Chapter 1, the use of an effective flocculant system in
paper manufacture increases production rates, improves paper quality and
reduces raw material requirements. A reduction in the quantity of organic
material in the effluent also improves environmental performance. In this
thesis, cationic polyacrylamide (CPAM) is combined with microparticles.
2.4.1. The mechanism of CPAM and microparticle dual polymer
systems for pulp flocculation
Cationic polyacrylamide (CPAM) is used widely as a drainage aid for all
types of chemical pulp but has been shown to be suitable for applications with a
high amount of fine fibres, such as in mechanical pulp (117). Bagasse pulp
similarly has a very high quantity of fine fibre and so CPAM was the polymer
selected for this study. CPAM’s mode of action is straightforward. The
cationic polymer attaches to the negatively charged surface of the fibres
resulting in neutralisation and flocculation.
The addition of anionic microparticles, such as bentonite or colloidal
silica can further improve flocculation by bridging the cationic flocculant
chains (see Figure 2.14).
Chapter 2 - Theory and Literature Review
53
Figure 2.14 Mechanism of silica microparticles (118).
The important difference between conventional polymer flocculation and
microparticle systems is that under conditions of high shear, such as those in a
papermachine headbox, the bonds formed with polymers are destroyed, but
microparticle systems have the ability to reflocculate the fibres after being
subjected to high shear.
2.4.2. Flocculant systems
A large volume of work has been undertaken in developing and
comparing chemical additives for various types of pulp (mainly wood grades)
(e.g. 107, 119, 120-126). The progression of additive chemistry has been from
the single polymer systems (pre 1970s) to dual polymer systems (1970s, 1980s)
to polymer and microparticle systems (1990s to present). The following
articles on flocculant research are all for wood pulp grades, often with high
amounts of very short fibre.
Hubbe (124) and Rojas & Hubbe (127) define three forms of chemicals
widely used as drainage additives: coagulants; flocculants; and microparticles.
Hubbe defines coagulants as compounds of high positive charge density which
act to neutralise the negative charge on fibres and ‘ionic trash’. Ionic trash is
undesirable very small fibres that are generated in mechanical pulping and has
very high surface area; bagasse pith may be considered ionic trash. Examples
of coagulants include aluminium sulphate (or alum), polyamines and
polyethyleneimine (PEI). Flocculants are polymers that link fine particles
together. Flocculants are often very high molecular weight copolymers of
acrylamide (PAM). Microparticles are very small negatively charged particles,
such as colloidal silica and bentonite, that interact with cationic flocculants (e.g.
Thomas J. Rainey, A study of bagasse pulp filtration
54
CPAM) or cationic starch and further improve flocculation. Brouillette and co-
workers (128), Sherman and Keiser (129) and Ledda et al. (130) all describe
various microparticle systems.
Flocculant systems often consist of various combinations of coagulants,
flocculants and microparticles. These studies tend to focus on fibre retention
rather than drainage. This point is noted by Allen and Yaraskavitch (119).
This study gives an excellent review of the dewatering potential of a large
number of systems. They make the following pertinent observations:
microparticle systems improve dewatering in alkaline systems; CPAM’s give a
small improvement in dewatering; and several other systems (e.g. PEI,
polyDADMAC and dual polymer) improve drainage at the expense of final
sheet moisture in vacuum dewatering. Britt and Unbehend (108) also observed
this effect.
Recently Carr (123, 131-133) has strongly advocated silica nanoparticles
rather than microparticles. Carr claims that the shear resistance of a particle
attached to a surface is inversely proportional to its size i.e. the smaller the
particle, the greater the shear resistance (131). Carr claims inventorship of
nanoparticles as a flocculant. However, Duffy (from Nalco Chemicals), in
1993, had previously noted that nanometer sized silica particles were extremely
efficient (118).
Miyanishi and Shigeru (134, 135) optimised flocculation and drainage by
comparing various microparticle systems and controlling the zeta potential (i.e.
the streaming potential which is an indication of the charge of the “white”
water). Miyanishi and Shigeru looked at the effect of adding various
chemicals, (alum, anionic polyacylamide, DADMAC, bentonite and CPAM), in
various sequences on both types of pulp containing and free from ionic trash. It
was found that alum, CPAM and then bentonite was the best sequence for acid
papermaking in the presence of ionic trash, with a 6% increase in flocculation
(as measured by improved turbidity) and 65% improvement in their defined
measure of “drainage”. DADMAC, anionic polyacrylamide and then bentonite
was found to be the best sequence for alkaline papermaking, with a 7%
improvement in flocculation and 50% improvement in “drainage”.
Chapter 2 - Theory and Literature Review
55
Kumar also used zeta potential to improve the retention of bagasse pulp
in a less thorough study (54). Using a DDJ, the bagasse pulp was fractionated.
The best order for retention aids was found to be rosin-starch-alum-filler. The
best zeta potential for retention was found to be -5 mV.
In contrast to the studies by Miyanishi and Shigeru (134, 135) and Kumar
(54), Britt (121) found that in dynamic systems, although zeta potential
provides additional information, flocculation can be improved without any
change in the zeta potential.
As can be observed from the variability in optimised flocculant systems,
the optimum chemical additives system is dependent on the pulp and needs to
be determined on a case-by-case basis. It appears from the literature that
CPAM and bentonite should give reasonable improvements in pulp drainage.
It is noted that most articles tend to focus on the fibre retention properties
of chemical additives rather than the drainage properties, which is the focus of
this thesis.
2.4.3. Literature on flocculants used for bagasse pulp
The literature on flocculants used for bagasse pulp is limited.
Abril’s work during the 1980s is the best reported literature with regards
to developing flocculant systems for improving the drainage behaviour of
bagasse pulp (39-41). Abril’s work was published in Spanish which was
translated into English because of its relevance to this study. Abril investigated
the effect of polymer drainage and retention aids on bagasse pulp (41). In this
laboratory study Abril used a DDJ to assess a range of drainage and retention
aids namely dextran, polyethyleneimine, anionic PAM and polyamideamine.
The polyethyleneimine and polyamideamine showed the biggest improvement
in retention and the best improvement in freeness.
In a further study (40), Abril tested modified polyamideamines, modified
polyethyleneimines (PEI’s) and CPAM in the laboratory. One of the modified
PEI’s gave the best drainage properties and was tested industrially. In the
industrial trial, the headbox freeness and retention (as measured by whitewater
Thomas J. Rainey, A study of bagasse pulp filtration
56
consistency) both improved, permitting the machine speed to be increased from
245-270 m/min to 300 m/min.
Ibrahem and co-workers (53) looked at PAM as a filler retention aid for
bagasse paper. The fillers investigated were titanium dioxide, silica and kaolin.
Strength data is provided for pulp containing each filler over a range of PAM
addition. PAM can improve filler retention by between 63% and 86%. It does
not contain information about the effect of PAM addition on drainage.
There is no known literature on the use of microparticles as a drainage aid
for bagasse pulp.
2.4.4. Using the Dynamic Drainage Jar as a tool for comparing
flocculants
The DDJ has been described in section 2.3.5.2. It was developed by Britt
and Unbehend in the early 1970s for comparing the effectiveness of flocculants
under the high shear conditions that exist in a paper machine. Pulp flocculants
can be tested very quickly in the laboratory using this equipment. The DDJ has
become the standard test method used by the paper industry to test the
suitability of pulp flocculants under high shear. Several TAPPI test methods
have been written that use this device.
In 1977, Unbehend, Britt’s colleague and frequent co-author, describes
the equipments use for measuring fines and colloidal retention. This paper
forms the basis of Tappi (the Technical Association of the Pulp and Paper
Industry) Test method T261 “Fines fraction by weight of paper stock by wet
screening”, making the equipment part of a standard test procedure.
In the standard test method, turbulence of the stock is maintained to
prevent pad formation (122). As shown in Figure 2.15, a stock is processed
through the DDJ giving the fines retention as a function of stirrer speed (line
A). When the same stock is processed with a strong dispersant, 50 ppm of
TAMOL 850 and the pH adjusted to 10.5 with sodium carbonate in the
presence of ultrasonic dispersion forces, a minimum fines retention line (line B)
is obtained, which is a characteristic of the stock. Britt proposed that the
Chapter 2 - Theory and Literature Review
57
colloidal forces are measured by the difference between line A and line B.
Flocculants raise the A line and dispersants lower it.
Figure 2.15 Retention in Dynamic Drainage Jar as a function of stirrer
speed (122).
From the literature, the DDJ is a reasonable approach to optimising a
chemical additives system.
2.4.5. Summary of chemical additives literature and theory
There is a large body of literature for pulp chemical additives. The
literature on bagasse pulp chemical additives is small and not recent. Although
microparticle systems are not very new, the effectiveness of these flocculant
systems are not reported for bagasse pulp.
The literature on the effect of chemical additives to improve the drainage
properties is not large as many studies focus solely on fines retention.
Miyanishi and Shigeru (134, 135), and Abril are the best works in measuring
the improvements in drainage caused by chemical additives. In every case,
these workers measure freeness rather than permeability which is a more
vigorous measure of drainage. As will be discussed, this thesis investigates the
compressibility and permeability of bagasse pulp. Quantifying the effect of
microparticle systems on the compressibility and permeability parameters of a
bagasse pulp has not been published before.
Stirrer speed, rpm
Fin
es r
eten
tio
n,
%
Thomas J. Rainey, A study of bagasse pulp filtration
58
No work exists quantifying the effects of flocculants on the permeability
and compressibility parameters required for modelling dynamic pulp
filtration/formation behaviour.
2.5. Summary of theory and literature review
The background literature for bagasse pulping has been discussed. The
work of Gartside and co-workers (28, 51, 65) stands out as the most thorough
work performed in Australia. However previous published work with bagasse
in Australia has traditionally focussed strongly on improving the pulp strength
properties and did not consider pulp filtration properties.
The properties of bagasse pulp have been reported by numerous workers
apart from Gartside and co-workers (e.g. 71). The fibre morphology and
chemical character have been described. The pulp physical properties depend
on the level of depithing.
The pulp permeability and compressibility theory has been described for
both steady-state filtration and dynamic filtration under compression. The
Kozeny-Carman equation is the most common steady-state permeability
correlation linking Darcy’s permeability factor, K, to porosity. The power-law
compression model is the most common steady-state model for pulp pads. The
dynamic filtration model developed in this thesis is based on an initially
networked filtration model (100). The equipment designs most commonly used
in pulp filtration studies have been discussed.
The method of testing the effectiveness of flocculants using a DDJ has
been presented, along with the mechanism of pulp fibre flocculation.
This study investigates the permeability and compressibility of bagasse
pulp which has not been performed extensively. The gaps in the literature have
been identified as foreshadowed in section 1.4. Particularly, this thesis adds to
the existing literature.
� The two options for treating bagasse prior to pulping
(fractionation and the mode of juice extraction) have not
Chapter 2 - Theory and Literature Review
59
previously been considered with a view to improving their
permeability and compressibility properties.
� Obtaining steady-state permeability data on steady-state
equipment and confirming the data with a second piece of
equipment is a unique approach.
� Quantifying the effect of flocculants on pulp pad steady-state and
dynamic permeability and compressibility has not been previously
studied.
� Finally, a dynamic filtration model has not been previously
investigated for a non-wood pulp such as bagasse pulp.
This study is the first time that the filtration of bagasse pulp has been
directly compared to wood pulp.
Thomas J. Rainey – A study of bagasse pulp filtration
60
Chapter 3
Experimental procedure and
modelling
The experimental component of the research plan was substantial. For the
filtration study three pieces of experimental equipment were constructed
specifically for the study, namely the ‘permeability cell’, the ‘compressibility cell’
and the modified DDJ (all terms are defined later in this chapter). This was in
addition to the three styles of digestion equipment used to produce the pulp
samples.
The research plan was implemented in six stages of experimentation and
modelling. This chapter proceeds in the order described by the experimental and
modelling methodology (section 3.1). The bagasse was treated and pulped using
the three types of digestion equipment, a ‘flow-through’ reactor, a ‘Parr’ reactor
and an ‘air-bath’ reactor (section 3.2). The chemical and physical properties of
the pulp were analysed as well as the fibre morphology (section 3.3). The steady-
state permeability of a pulp pad was measured using a custom built ‘permeability
cell’ (section 3.4). The steady-state compressibility of a bagasse pulp pad was
measured using a custom built ‘compressibility cell’ (section 3.5). The steady-
state permeability and compressibility of bagasse pulp was compared to numerous
benchmark pulp samples and the findings of previous workers for wood pulp.
The steady-state permeability and compressibility parameters for bagasse pulp
pads were used in a dynamic model which was coded in FORTRAN and
compared to experimental data obtained under dynamic filtration conditions
Chapter 3- Experimental procedure and modelling
61
(section 3.6). A suitable chemical additive system was optimised with a modified
Dynamic Drainage Jar (DDJ) using a bagasse pulp slurry, and the effect of
vacuum on drainage time was examined. The modified DDJ was also used to
obtain complementary information about the drainage behaviour through thin pulp
mats rather than thick pulp pads. The effect of chemical additives on the steady-
state permeability and compressibility constants of a bagasse pulp pad was
quantified (section 3.7). Finally a summary of the experimental procedure is
presented (section 3.8).
3.1. Overview of experimental and modelling methodology
The aims and objectives were achieved in six phases using the following
program of work.
Phase 1 Fractionated bagasse pulp from milled and diffuser bagasse
was prepared. A benchmark Australian eucalypt pulp and a
commercial bagasse pulp were obtained (section 3.2);
Phase 2 The physical and chemical properties of the bagasse pulp
were determined (section 3.3);
Phase 3 The steady-state permeability parameters of a bagasse pulp
pad were determined using simple permeability
experimental equipment. The effect of bagasse fraction and
the mode of juice extraction on pulp pad permeability was
examined without the addition of flocculants (section 3.4);
Phase 4 The steady-state compressibility parameters of a bagasse
pulp pad were determined using simple compression
equipment. The effect of bagasse fraction and the mode of
juice extraction on pulp pad compressibility was examined,
also without flocculants added (section 3.5);
Phase 5 The values of the steady-state permeability and
compressibility parameters were used by a dynamic model
for predicting the solidity and consequently load pressure of
Thomas J. Rainey – A study of bagasse pulp filtration
62
a pulp pad compressed under dynamic conditions. The
model values are compared to data from dynamic filtration
experiments (section 3.6);
Phase 6 A suitable chemical additives system is optimised using a
modified DDJ using a pulp slurry rather than a pulp pad as
used in phases 3-5. The effect of additives on fines
retention and drainage time is determined. The effect of
chemical additives on the steady-state parameters of a
bagasse pulp pad is determined by repeating phases 3, 4 and
5 above (section 3.7).
The above order of research is used in the experimental procedure section
(Chapter 3) and the results section (Chapter 4).
3.1.1. Preparation of Australian bagasse pulp
Bagasse was prepared in a manner to maximise its permeability properties
and permit its long term storage for this study.
For Objective 1a, bagasse was separated into three fractions prior to pulping
using two wire mesh sieves of different aperture sizes (12.5 mm and 4 mm). The
three bagasse sizes produced were nominated: ‘coarse’, ‘medium’ and ‘fine’ pith
material. The terms ‘coarse’ bagasse pulp and ‘medium’ bagasse pulp are used
extensively in this thesis and refer to pulp which originated from the coarse and
medium fractions of bagasse respectively. Pulp from the ‘fine’ material blocks
the pores of the paper mat as it forms, reducing the drainage rate and sheet quality
(e.g. poor formation and wire-marks). Removing as much ‘fine’ material as
possible prior to pulping improves the drainage properties of the mat.
For Objective 1b, samples of bagasse from the different modes of juice
extraction were collected (i.e. milled and diffuser bagasse) from the same factory.
Several other pulp samples were prepared or obtained for comparative
purposes. The most important comparative pulp samples are: Eucalyptus globulus
pulp; pulp produced from Argentinean depithed bagasse used at a commercial
pulp mill; and a conventionally depithed Australian bagasse pulp.
Chapter 3- Experimental procedure and modelling
63
3.1.2. Physical and chemical property testing
The Australian bagasse pulp samples were evaluated for their chemical
properties. The chemical analyses of the pulp samples included carbohydrate
composition by High Performance Liquid Chromatography, Klasson and acid
soluble lignin, ash, extractives and pulp yield.
Bagasse is commonly used for the production of linerboard, writing paper
and tissues amongst other products. The pulp samples were evaluated for strength
properties (tensile, tear, burst and short-span compression) over a range of
refining levels, fibre length distribution and optical properties amongst other
properties. The suitability of the bagasse pulp produced in this study for these
grades were assessed.
The pulp fibres were thoroughly measured for their morphology including
the distributions of fibre length, using a Kajaani fibre length analyser, as well as
other parameters including wall thickness and collapse ratio using a confocal laser
microscope.
3.1.3. Steady-state permeability property testing
The effect of bagasse preparation on the steady-state permeability properties
of pulp pads was studied. Objective 1a and Objective 1b (section 1.3) were
investigated with respect to the steady-state permeability properties.
Pulp permeability was measured in a simple experimental apparatus referred
to as a ‘permeability cell’. A transparent Perspex tube filled with pulp and
attached to a constant head tank was used to achieve steady-state flow.
Using this simple equipment, the suitability of the Kozeny-Carman
permeability model could be quickly determined.
The variables measured in the steady-state testing are the pulp specific
surface area, Sv, and the swelling factor, �. These parameters were determined for
use in the Kozeny-Carman permeability model (78, 79). These steady-state
variables are required for the dynamic filtration model. The findings of this
permeability study are compared to that of previous workers for wood pulp, as
Thomas J. Rainey – A study of bagasse pulp filtration
64
well as to the only known previous work on bagasse permeability which was
performed recently (64).
The optimum values of Sv and � depend on whether a constant or a variable
Kozeny factor, ‘k’ is used. For this study, both constant and variable k was used.
Ingmanson and co-workers (81) found that using a variable Kozeny factor
resulted in an increase in the prediction for � of around 25% and a decrease in the
prediction for Sv of around 7% for wood pulp. The variation in Sv and � is
measured for non-wood pulp.
Objective 1a and Objective 1b were investigated using Student’s t-test with
respect to their effect on the compressibility properties Sv and �.
The steady-state permeability of eucalypt pulp, pine pulp and Argentinean
bagasse pulp was also measured.
3.1.4. Steady-state compression testing
The steady-state compressibility behaviour of pulp pads was measured using
simple compression equipment, i.e. a ‘compressibility cell’, using a simple Power-
Law correlation between load pressure and pulp concentration, Ps = M C N (see
Chapter 2 for definitions).
The pulp pad was initially compressed over a very long time-period to
measure the quasi steady-state compressibility parameters. The steady-state
factors M and N are necessary for the dynamic filtration modelling. Objective 1a
and Objective 1b were investigated using Student’s t-test with respect to their
effect on the compressibility properties M and N.
The same samples tested for their steady-state permeability were also
measured for their steady-state compressibility and compared to the numerous
benchmark pulp samples used in this study, including eucalypt, as well as
previous workers.
3.1.5. Dynamic filtration modelling and verification
In dynamic filtration, the permeability properties change as the pulp pad
compresses. Once the steady state compressibility and permeability parameters
Chapter 3- Experimental procedure and modelling
65
are determined, the dynamic filtration of bagasse pulp can be predicted using a
filtration model similar to that used for wood pulp (100). This model requires the
steady-state permeability parameters, Sv and �, and the compressibility
parameters, M and N, for bagasse pulp previously determined experimentally.
The model was non-dimensionalised and coded in FORTRAN. The model
predicts the dynamic filtration behaviour using the experimentally determined
steady-state permeability and compressibility parameters. The actual dynamic
filtration behaviour of the pulp is then measured experimentally in the
compressibility cell. The experimental data is compared with the predictions of
the dynamic model in order to verify the model.
3.1.6. Effect of chemical additives on the drainage and retention
properties
A chemical additives system is optimised using a Dynamic Drainage Jar.
This was performed using a pulp slurry. The equipment was modified to also
investigate the effect of vacuum on fine fibre retention and drainage time.
In previous phases of this study, thick pulp pads were investigated because
the permeability and compressibility can be determined with simple equipment.
The behaviour of pulp pads is frequently used by numerous workers to represent
the behaviour of thin pulp mats (e.g. 81, 82, 83, 84-86). In this phase, the
modified DDJ was also used to obtain additional information on the behaviour of
thin bagasse pulp mats, which more closely resembles a Fourdrinier former than a
Twin-wire former.
Finally, the effect of chemical additives on pulp pad steady-state and
dynamic permeability and compressibility is quantified.
3.1.7. Flow diagram of the experimental and modelling methodology
The relationship between sections of the experimental and modelling
methodology is shown in Figure 3.1. The numbers in the figure are the phases of
the methodology described at the start of section 3.1. This is a theme of this
thesis. Chapter 3, the experimental and modelling procedure, and Chapter 4, the
results and discussion, proceed in the same order as the methodology.
Thomas J. Rainey – A study of bagasse pulp filtration
66
Figure 3.1 Flow diagram of the experimental and modelling methodology
for bagasse pulp.
Phase 1. Bagasse pulp preparation
Steady-state experiments with bagasse pulp pads
Phase 3. Steady-state
permeability
experiments
Obtain Sv and �
Phase 4. Steady-state
compressibility
experiments
Obtain M and N
Bagasse pulp
Phase 5. Dynamic filtration modeling and experiments
with bagasse pulp pads
Dynamic filtration model
Use steady-state
parameters, Sv, �, M and N
Dynamic
compression
experiments
Model
verification
Phase 6.
Development of a
chemical additives
system using a
pulp slurry
Phase 2. Physical
and chemical
property testing
Bagasse preparation
Fractionated bagasse ‘coarse’ vs ‘medium’ pulp from mill or diffuser
(Chemical characterisation only)
Bagasse pulp slurry
With and without flocculants
Bagasse pulp pad
Chapter 3- Experimental procedure and modelling
67
3.2. Bagasse pulp preparation
The preparation and storage of bagasse and pulp created some challenges
since bagasse is extremely bulky with a specific mass of 150 kg/m3. Bagasse also
degrades quickly due to the presence of a residual sugar. It was necessary to wash
it and dry it as quickly as possible for long-term storage in a large walk in
refrigerator at 4 °C. The large number of pulp samples generated in this report are
summarised in Appendix B.
The treatment of the bagasse prior to pulping is presented in section 3.2.1.
The pulp samples were prepared in Melbourne and QUT using three types of
reaction equipment as described in section 3.2.2. The statistical methods used to
determine whether there is a difference between populations of pulp samples are
provided in section 3.2.3. The effect of bagasse pre-treatment on yield and kappa
number is provided in section 4.1. The physical and chemical properties of the
pulp are provided in section 4.2.
3.2.1. Collection of raw materials
3.2.1.1. Australian bagasse
As previously mentioned, bagasse was collected from both a sugar diffuser
and a sugar mill and fractionated into three fractions ‘coarse’, ‘medium’ and ‘fine’
fractions.
The pre-treatment procedure used in this study is intended to maximise the
permeability of Australian bagasse pulp pads and also to minimise degradation of
the bagasse for long term storage. The total amount of pith removed (around
43%) was higher than normally used by industry to achieve acceptable bagasse
pulp permeability (typically 30%).
Bagasse was collected from CSR Invicta sugar factory. The Invicta milling
train consisted of a shredder and five milling units including the final dewatering
mill. The Invicta diffuser consisted of a separate shredder, a preliminary milling
unit, the diffuser and a final dewatering mill.
Thomas J. Rainey – A study of bagasse pulp filtration
68
On 28th September 2006 ten 75 L bins were lined with garbage bags. Six
bins were filled with bagasse from the final dewatering mill of the milling train.
Four were filled with bagasse from the final dewatering mill following the
diffuser. Each bin was filled with 10 kg of bagasse. The bagasse in these bins
were turned over several times in order to reduce the temperature and moisture
content and hence degradation during transport. The bins arrived at QUT,
Carseldine Campus on 4th October 2006.
The bagasse obtained from the sugar mill was from cane species Q208B (B
for burnt). The bagasse collected from the diffuser was TellB. It was not possible
to collect bagasse of the same variety of cane from both the mill and the diffuser
during the visit. As will be shown in Chapter 4, the difference in cane varieties
was inconsequential. No difference was found in the pulping kinetics (section
4.1.1) or the permeability and compressibility characteristics (sections 4.3 and
4.4).
The fibre content of the parent cane was measured by factory staff and
determined to be 15.6% (wet basis) for both varieties of cane. The fibre content
of Australian cane is typically 10% to 17%. The fibre content of the cane was
towards the higher end of this range.
The bagasse was washed in copious amounts of water to remove sugar using
a cement mixer. The bagasse mixture was drained through a 4 mm wire mesh.
The fines in the filtrate were recovered by refiltering the filtrate through the
bagasse bed several times. Only 3% of the fines were lost through this washing
process.
The bagasse was allowed to dry to 10% moisture, see Figure 3.2. The
bagasse piles were rotated with one another for even exposure to the sun.
Chapter 3- Experimental procedure and modelling
69
Figure 3.2 Photograph of bagasse drying outside on tarpaulins.
The washed milled and diffuser bagasse was separated into three fractions
prior to pulping using two wire mesh sieves of different aperture sizes, 12.5 mm
and 4 mm respectively. Subsamples of around 50 g of bagasse were manually
sieved for approximately 3 min to achieve the separation. The three bagasse sizes
produced were nominated: ‘coarse’ which accounts for the 25 % of the bagasse
that is retained on the 12.5 mm sieve (i.e. +2 mesh); ‘medium’ (i.e. 4.0 mm to
12.5 mm) which accounts for the 35% of the bagasse that passes the 12.5 mm
sieve but is retained on the 4.0 mm sieve (i.e. +6 mesh); and ‘fine’ which accounts
for around 40 % of the bagasse and passes through the 4.0 mm sieve (i.e. -6
mesh).
The fractionated bagasse samples are shown in Figure 3.3 (a), (b) and (c)
together with samples of ‘whole’ (unfractionated) Australian bagasse (d). The
‘coarse’ bagasse (a) contains a much higher content of large chip-like material
compared to the ‘medium’ bagasse (b). These definitions of ‘coarse’, ‘medium’,
Bin 1 Bin 2
Bin 3 Bin 4
Bin 5 Bin 6
Bin 7 Bin 8
Bin 9 Bin 10
Thomas J. Rainey – A study of bagasse pulp filtration
70
‘fine’ and ‘whole’ bagasse pulp are used throughout this thesis. The ‘fine’ fraction
is assumed to be mainly pith material so this terminology is used interchangeably.
Figure 3.3 Photographs of (a) ‘coarse’, (b) ‘medium’ and (c) ‘fine’
fractions of Australian bagasse, and (d) Australian ‘whole’
bagasse.
A sample of Australian bagasse was sieved in order to remove 30% of its
shortest material. This is typical of overseas industrial depithing operations. The
pulp produced from this Australian bagasse is herein referred to as ‘30% depithed’
bagasse pulp.
After washing and fractionating, the bagasse was then stored in a walk-in
fridge (4 °C) until it was ready to be pulped.
3.2.1.2. Argentinean bagasse
A depithed Argentinean bagasse sample that is used by the company
Ledesma Paper Mill to make writing papers was included in the evaluation. Their
depithing process removed 30% of the finest material (i.e. the pith). This sample
50 mm 50 mm
50 mm
(a) (b)
(c) (d)
50 mm
Chapter 3- Experimental procedure and modelling
71
was stored in the fridge but not washed so as not to alter its preparation conditions
prior to pulping. The pulp produced from this sample is referred to as
‘Argentinean’ bagasse pulp.
3.2.1.3. Wood material
Samples of Eucalyptus globulus and Pinus radiata wood material were
supplied in pulp form by Ensis and the Australian Pulp and Paper Research
Institute (APPI) respectively. These organisations are two of Australia’s leading
pulp and paper research and development companies. No pre-treatment was
performed on these samples. The pulping conditions are provided in section
3.2.2.3.
3.2.2. Pulp sample preparation
A large number of pulp samples were required to (i) investigate
permeability and compressibility differences between pulp samples originating
from different size fractions of bagasse2 as well as differences in milled and
diffuser bagasse pulp, (ii) undertake the required physical property testing and (iii)
investigate the effect of chemical additives on the flocculation of pulp fibres.
The large number of pulp samples was particularly important to achieve
Objective 1. If the populations were compared with a small number of large
cooks, then subtle changes in cooking conditions between the cooks could
potentially affect the outcome. To test whether there is a difference between
fractionated milled and diffuser bagasse, samples were pulped in an APPI digester
containing six 1.5 L cells (discussed in more detail in section 3.2.2.1). This
reactor is called a ‘flow-through’ digester herein. Samples of bagasse were
cooked in a randomised order.
Several pulp samples were produced in a much larger batch 18.5 L reactor
for a number of reasons. The size of the samples produced in the APPI ‘flow-
through’ digester were not sufficient for destructive physical property testing
(section 3.3). Also, for experiments involving chemical additives (section 3.7),
2 For bagasse with a very high proportion of short fibres, it was not possible to pulp the material because it was difficult to circulate the liquor in the APPI ‘flow-through’ digester.
Thomas J. Rainey – A study of bagasse pulp filtration
72
the pulp had to be disposed after each experiment. As such, a much larger batch
of pulp was prepared so that subtle differences that may affect pulping
experiments could be eliminated as a potential source of error.
Over 60 pulp samples were produced in the course of the project. Each
bagasse pulp sample produced was labelled with a unique number which is
referred to hereafter in this thesis. The pulping conditions, origin of the bagasse,
yield and kappa number for each pulp sample are provided in Appendix B.
Supplementary photographs of the pulping equipment are provided in
Appendix C.
3.2.2.1. Bagasse pulping in the ‘flow-through’ digester
The APPI ‘flow-through’ digester consists of six cells into which bagasse is
packed. Each cell is 1.5 L. Hot cooking liquor is pumped from a 50 L tank
through each of the cells and drains back into the tank. The flow diagram of the
equipment is reproduced in Figure 3.4 (136) and a photograph of the equipment is
provided in Figure 3.5.
Figure 3.4 Sketch of the 6 cell ‘flow-through’ digester at APPI (136).
Digestion cells
Liquor inlet lines
Chapter 3- Experimental procedure and modelling
73
Figure 3.5 Photograph of the APPI ‘flow-through’ digester showing the 6
digestion cells.
Bagasse was soaked in warm water for 20 min to soften it so that each 180 g
sample of ‘medium’ bagasse and 210 g of ‘coarse’ bagasse could be packed into
the digester cells. The increase in flexibility is due to the plasticisation of the
lignin rather than bending of the sclerenchyma pulp fibres.
Fifty litres of cooking liquor was recirculated through six cells containing
100-200 g of fractionated bagasse (air dry basis). The pulping conditions were 0.4
M sodium hydroxide (approx. 13.8% Na2O on oven dry fibre) and 0.1%,
anthraquinone, AQ, (on oven dry fibre) at 145 °C.
In this reactor, the cells can be independently isolated from the cooking
liquor by manual valves. The liquor is heated indirectly by steam. When the
liquor reaches temperature, the liquor can be circulated immediately through the
material in the cell.
An initial kinetics study was performed to determine the cooking time
required to achieve a pulp with a kappa number (i.e. residual lignin content) of 20.
A pulp screen was not available during the trials with the APPI digester, so the
kappa number was measured on unscreened pulp. The cooking length was varied
between 5 min and 70 min. It was found that only 30 min of cooking time was
Cells
Inlet liquor lines from common header (valve on each)
Outlet liquor
lines
Thomas J. Rainey – A study of bagasse pulp filtration
74
required. Normally several hours is required to produce wood pulp using this
equipment (136). Bagasse pulp is well known to delignify much more quickly
than wood chips due to the high reactivity of grass lignins.
The depithed bagasse obtained from Argentina’s Ledesma Mill was also
pulped under these conditions for 30 min using the ‘flow-through’ reactor.
At the end of a cook, the pulp was transferred to a standard disintegrator.
The pulp was disintegrated for 10,000 rev. The pulp was thoroughly washed with
water and dewatered using a very large steel Buchner funnel.
A total of 30 pulp samples were generated in the ‘flow-through’ digester.
3.2.2.2. Bagasse pulping in a batch ‘Parr reactor’
It was not possible to pulp whole (unfractionated) bagasse or the ‘fine’
fractionated material in the APPI ‘flow-through’ digester because the liquor
would pool on top of the bagasse and not permeate through the bed of bagasse.
Consequently, samples of whole and fine Australian bagasse were pulped to a
target kappa number of 20 in the 18.5 L batch reactor at 170 °C for 105 min at a
liquor to fibre ratio of 14:1 with a concentration of approximately 0.4 M sodium
hydroxide and 0.1% AQ. The Parr reactor is shown in Figure 3.6. The reactor is
electrically heated. The time to temperature for the majority of the experiments
was typically 45 min - 60 min. 1 kg of bagasse (10%-15% moisture) was loaded
into the Parr reactor in each cook. The ‘Parr reactor’ is cooled by ambient water
flowing through serpentine cooling coils and takes 60 min to reach a temperature
at which the vessel can be safely handled.
Chapter 3- Experimental procedure and modelling
75
Figure 3.6 The QUT 18.5 L Parr reactor.
In addition to the ‘whole’ and ‘fine’ bagasse pulp samples, large quantities
of bagasse pulp originating from ‘coarse’ and ‘medium’ fractions of bagasse were
produced in this reactor for physical property testing and the tests involving
chemical additives.
Pulp produced from ‘30% depithed’ bagasse was also produced in this
reactor under these cooking conditions. This pulp is used for benchmarking
purposes as it has been depithed to a similar level to that used by overseas
commercial operations. This benchmark pulp was cooked in this reactor so as to
prevent any pith material from being washed into the liquor. It is acknowledged
that this benchmark pulp sample was cooked at a higher temperature than the
majority of the pulp samples cooked using the ‘flow-through’ reactor. It was
decided that preserving the pith in this benchmark bagasse pulp sample, as could
be achieved using the Parr reactor, was very important.
In order to determine whether bleaching had any effect on the permeability
and compressibility properties, one of the pulp samples produced using the ‘Parr
reactor’ (Sample 56) was bleached using calcium hypochlorite according to Tappi
test method UM-206 from 28 brightness to 54 brightness (137).
Crane for moving
vessel head
Heating
jacket
Temperature
controller
Vessel head
Thomas J. Rainey – A study of bagasse pulp filtration
76
3.2.2.3. Benchmark wood pulp
The two wood pulp samples were supplied by Australian pulp and paper
research and development organisations.
Eucalypt pulp was prepared at Ensis, Melbourne, Australia, using an ‘air-
bath’ reactor. The wood is loaded into a sealed cell with 2 L volume and the cell
is loaded into a pressure vessel and heated with steam. The cooking conditions
used to produce the pulp were 11.75% Na2O on oven dry fibre, sulphidity of 25%,
cooking temperature of 165oC for 2 h. The eucalypt was pulped to a target of 20
kappa.
A sample of kraft pine pulp was obtained from APPI, also in Melbourne.
The sample was similarly prepared in an air-bath reactor and pulped to a kappa
number of 20 using kraft pulping chemicals. The concentration of the cooking
chemicals that were used is not known.
3.2.2.4. Pulp screening
Each pulp sample was screened through a 200 �m slotted Packer screen
with water recirculation. The pulp samples were not allowed to dry at any stage.
The pulp was placed in a dough mixer to break up the pulp in order to make the
bagasse pulp (Sample 60, Figure 4.30). In these experiments, the effect of
chemical additives was also investigated as a function of vacuum. The chemicals
added were 0.05% CPAM and 0.06% bentonite.
Thomas J. Rainey – A study of bagasse pulp filtration
148
0%
10%
20%
30%
40%
50%
60%
70%
80%
90%
100%
0 10 20 30 40 50
Vacuum kPa
Fin
es r
ete
ntion
0
10
20
30
40
50
60
70
80
90
100
Dra
ina
ge t
ime
(s)
Fines retention 'depithed' bagasse pulp no additives
Fines retention 'depithed' bagasse pulp 0.05% CPAM + 0.06% bentonite
Drainage time 'depithed' bagasse pulp no additives
Drainage time 'depithed' bagasse pulp 0.05% CPAM + 0.06% bentonite
Read from right hand axis
Read from left hand axis
Figure 4.28 The effect of vacuum and chemical additives on the fines
retention and drainage time of a ‘30% depithed’ bagasse pulp
(Sample 58), 1000 rpm shear.
0%
10%
20%
30%
40%
50%
60%
70%
80%
90%
100%
0 10 20 30 40 50 60 70
Vacuum kPa
Fin
es r
ete
ntion
0
10
20
30
40
50
60
70
80
90
100
Dra
ina
ge t
ime
(s)
Fines retention no additives Fines retention 0.05% CPAM + 0.06% bentonite
Drainage time no additives Drainage time 0.05% CPAM + 0.06% bentonite
Read from right hand axis
Read from left hand axis
Figure 4.29 The effect of vacuum and chemical additives on the fines
retention and drainage time of a ‘coarse’ bagasse pulp (Sample
56), 1000 rpm shear.
Chapter 4 - Results and discussion
149
0%
10%
20%
30%
40%
50%
60%
70%
80%
90%
100%
0 10 20 30 40 50
Vacuum kPa
Fin
es r
ete
ntion
0
10
20
30
40
50
60
70
80
90
100
Dra
ina
ge t
ime
(s)
Fines retention no additives Fines retention 0.05% CPAM + 0.06% bentonite
Drainage time no additives Drainage time 0.05% CPAM + 0.06% bentonite
Read from right hand
axis
Read left right hand axis
Figure 4.30 The effect of vacuum and chemical additives on the fines
retention and drainage time of a ‘medium’ bagasse pulp
(Sample 60), 1000 rpm shear.
For each bagasse pulp examined, the CPAM/bentonite system improved the
retention of fines and the drainage time.
These data provided an unexpected result. When no chemical additives
were used, the ‘30% depithed’ bagasse pulp (Sample 58) initially had a longer
drainage time than the ‘coarse’ bagasse pulp (Sample 56) but as the vacuum
increased, the drainage time of the ‘depithed’ bagasse pulp improved and the
drainage time became quicker than the ‘coarse’ bagasse pulp. This effect is
shown in Figure 4.31. At a vacuum level greater than 10 kPa, the ‘30% depithed’
bagasse pulp drained more quickly than the ‘coarse’ bagasse pulp.
Thomas J. Rainey – A study of bagasse pulp filtration
150
0%
10%
20%
30%
40%
50%
60%
70%
80%
90%
100%
0 10 20 30 40 50 60 70Vacuum kPa
Fin
es r
ete
ntion
0
10
20
30
40
50
60
70
80
90
100
Dra
ina
ge t
ime
(s)
Fines retention 'Coarse bagasse' Test 56 no additives Fines retention 'Depithed bagasse' Test 58 no additives
Drainage time 'Coarse bagasse' Test 56 no additives Drainage time 'Depithed bagasse' Test 58 no additives
No additives
Read from right hand axis
Read from left hand axis
Figure 4.31 The effect of vacuum on the fines retention and drainage time
of ‘coarse’ (Sample 56) and ‘30% depithed’ (Sample 58)
bagasse pulp, 1000 rpm shear, no flocculants added.
This effect was exacerbated when chemical additives were used (0.05%
CPAM and 0.06% bentonite). At 5 kPa, the ‘30% depithed’ bagasse pulp had
faster drainage than the ‘coarse’ bagasse pulp.
Chapter 4 - Results and discussion
151
50%
55%
60%
65%
70%
75%
80%
85%
90%
95%
100%
0 10 20 30 40 50
Vacuum kPa
Fin
es r
ete
ntion
0
10
20
30
40
50
60
70
80
90
100
Dra
ina
ge t
ime
(s)
Fines retention 'Coarse bagasse' with additives Fines retention 'Depithed bagasse' with additives
Drainage time 'Coarse bagasse' with additives Drainage time 'Depithed bagasse' with additives
With additives
Read from right hand axis
Read from left hand axis
Figure 4.32 The effect of vacuum on the fines retention and drainage time
of ‘coarse’ (Sample 56) and ‘30% depithed’ (Sample 58)
bagasse pulp, 1000 rpm shear, with flocculants added.
It was anticipated that the drainage time of the ‘coarse’ bagasse pulp under
vacuum and shear conditions would be quicker than the ‘depithed’ bagasse pulp
based on its substantially lower Sv alone (~4600 cm-1 for ‘30% depithed’ bagasse
pulp compared to ~1500 cm-1 for ‘coarse’ bagasse pulp). This affect was also
observed for ‘medium’ bagasse pulp. Fibre to fibre interactions during
compression evidently plays an important role during bagasse pulp pad formation
in the DDJ. This is explored further in the next section.
4.6.3. The effects of chemical additives on permeability and
compressibility parameters
4.6.3.1. The effect of chemical additives on bagasse pulp permeability parameters
The study into the permeability properties of bagasse pulp was revisited
using the CPAM/bentonite additive system. The experimental procedure used in
section 3.4 was repeated with the exception that the chemical additives were
added to the pulp slurry prior to loading into the cell. Figure 4.33 and Figure 4.34
show the graph of (Kc2)1/3 against c for a ‘coarse’ and ‘medium’ bagasse pulp
respectively. As can be observed from the figures, there was not found to be a
Thomas J. Rainey – A study of bagasse pulp filtration
152
statistically significant difference in the slope or intercept of these plots and
consequently no difference in Sv or �. This was confirmed using Student’s t-test
with a 95% confidence interval, using the pooled estimate of standard deviation
from section 4.3.
0
0.0002
0.0004
0.0006
0.0008
0.001
0.0012
0.0014
0.0016
0.0018
0.00 0.05 0.10 0.15 0.20
Concentration, c (g/cm3)
(Kc
2)1
/3
Sample 43 no additives
Sample 43 with additives
Figure 4.33 The effect of chemical additives on the permeability of a
‘coarse’ bagasse pulp (Sample 43).
Chapter 4 - Results and discussion
153
0
0.0002
0.0004
0.0006
0.0008
0.001
0.0012
0.0014
0.0016
0.00 0.05 0.10 0.15 0.20
Concentration, c (g/cm3)
(Kc
2)1
/3
Sample 18 no additives
Sample 18 with additives
Linear (Sample 18 no additives)
Figure 4.34 The effect of chemical additives on the permeability of a
‘medium’ bagasse pulp (Sample 18).
The effect of additives was significant for a ‘30% depithed’ bagasse pulp
(Sample 58), see Figure 4.35. ‘30% depithed’ bagasse pulp had higher
permeability when chemical additives were used, although the permeability was
still lower than that of eucalypt (without additives). This was confirmed at a 95%
confidence interval.
Thomas J. Rainey – A study of bagasse pulp filtration
154
0
0.0002
0.0004
0.0006
0.0008
0.001
0.0012
0.0014
0.000 0.050 0.100 0.150 0.200
Concentration, c (g/cm3)
(Kc
2)1
/3
Eucalypt pulp, no additivesWhole bagasse pulp, no additives'Depithed' bagasse pulp, no additives'Depithed' bagasse pulp, with additivesLinear (Eucalypt pulp, no additives)
Figure 4.35 The effect of chemical additives on the permeability of a ‘30%
depithed’ bagasse pulp (Sample 58).
The results for Sv and � are shown in Table 4.19. The chemical additives
had a strong affect on the ‘30% depithed’ bagasse pulp, greatly reducing its Sv but
not on the ‘coarse’ or ‘medium’ bagasse pulp. There was not found to be any
statistically significant difference in � for any bagasse pulp sample using a 95%
confidence interval.
Table 4.19 Effect of additives on the permeability parameters Sv and �.
Parameter Bagasse pulp type No additives With additives
Coarse (Sample 43) 3.44 3.45
Medium (Sample 18) 3.33 2.98
� (-)
�PESD, �=0.216
30% depithed (Sample 58) 2.97 3.26
Coarse (Sample 43) 1540 1580
Medium (Sample 18) 1820 2080
Sv (cm-1)
�PESD, Sv=211 cm-1
30% depithed (Sample 58) 4640 3060
Chapter 4 - Results and discussion
155
4.6.3.2. Effect of chemical additives on bagasse pulp compressibility parameters
The investigation into the steady state compressibility of bagasse pulp in
section 4.4 was revisited using the CPAM/bentonite system. Figure 4.36 and
Figure 4.37 shows typical results for a ‘coarse’ and ‘medium’ bagasse pulp
respectively. In both figures, the data is shown prior to chemical additives and
after the addition of chemical additives. There was no difference in the steady-
state compression factors M and N.
Figure 4.36 The effect of chemical additives on the quasi steady-state
compression of ‘coarse’ bagasse pulp (Sample 20).
Thomas J. Rainey – A study of bagasse pulp filtration
156
Figure 4.37 The effect of chemical additives on the steady-state
compression of ‘medium’ bagasse pulp (Sample 18).
The effect of chemical additives is more pronounced on the steady state
compressibility of a ‘30% depithed’ bagasse pulp (Figure 4.38). This mirrors the
observation that chemical additives only affect the permeability parameters of a
‘depithed’ bagasse pulp.
Chapter 4 - Results and discussion
157
Figure 4.38 The effect of chemical additives on the steady-state
compression of ‘depithed’ bagasse pulp.
Typical results for the steady-state compressibility test are shown in Table
4.20. The results were duplicated using other pulp samples (Sample 26, a ‘coarse’
bagasse pulp and Sample 42, a ‘medium’ bagasse pulp). The only statistically
significant result is that the chemical additives system affected the ‘depithed’
bagasse pulp compressibility parameters by increasing both M and N.
Table 4.20 Typical effect of chemical additives on bagasse pulp
compressibility parameters.
Parameter Bagasse pulp type No additives With additives
Coarse (Sample 20) 3.77 3.93
Medium (Sample 18) 3.79 3.79
Log M (kPa)
�PESD, m=0.15
30% depithed (Sample 58) 3.14 3.74
Coarse (Sample 20) 2.76 2.98
Medium (Sample 18) 2.73 2.73
N, -
�PESD, n=0.12
30% depithed (Sample 58) 1.89 2.65
Thomas J. Rainey – A study of bagasse pulp filtration
158
4.6.4. The effect of chemical additives on bagasse pulp’s dynamic
filtration behaviour
In section 4.6.3, it was found that chemical additives affected the Sv, M and
N of only the ‘30% depithed’ pulp. The ‘coarse’ and ‘medium’ bagasse pulp
could not be shown, statistically speaking, to be affected by chemical additives.
The dynamic filtration tests in section 3.6 were revisited to look at the effect
of chemical additives. The results for ‘coarse’ pulp (Sample 43), ‘medium’ pulp
(Sample 18) and ‘30% depithed’ pulp (Sample 58) are shown in Figure 4.39.
Similar results for the ‘coarse’ and ‘medium’ pulps were obtained using Sample
26 (a ‘coarse’ pulp) and Sample 42 (a ‘medium’ pulp).
Under dynamic conditions, the governing equation (Equation 2.25) is
dominated by the flexural term D(�), reproduced below. In the case of the ‘30%
depithed’ bagasse pulp, the reduced load has been caused by an increase in the
permeability term (K(�)) and compression term (mn �n-1).
( ) ( ) ( )�
mnK 1D
1n−φφφ−φ=φ
Although it could not be determined experimentally using a 95% confidence
interval that the chemical additives affect either the steady-state permeability or
compressibility properties of ‘coarse’ or ‘medium’ bagasse pulp, there was some
anecdotal evidence of their effect demonstrated in the dynamic experiment, Figure
4.39. The figure shows that there is a small reduction in load pressure when
chemicals are added to the ‘medium’ pulp.
159
0
10
20
30
40
50
60
70
80
90
100
0 0.02 0.04 0.06 0.08 0.1 0.12 0.14 0.16 0.18 0.2
Concentration (g/cm3)
Pre
ssu
re (
kP
a)
'Coarse' bagasse pulp (Sample 43) no additives 'Coarse' bagasse pulp (Sample 43) with additives
'Medium' bagasse pulp (Sample 18) no additives 'Medium' bagasse pulp (Sample 18) with additives
'Depithed' bagasse pulp, (Sample 58) no additives 'Depithed' bagasse pulp (Sample 58) with additives
'30% Depithed' pulp
'Medium' pulp
'Coarse' pulp
Figure 4.39 The effect of chemical additives on the dynamic filtration of ‘depithed’, ‘coarse’ and ‘medium’ bagasse pulp.
159
Chap
ter 4 –
Resu
lts and d
iscussio
n
Thomas J. Rainey – A study of bagasse pulp filtration
160
Let us now re-examine the finding in section 4.6.2 where the ‘30%
depithed’ bagasse pulp had faster drainage under vacuum in the modified DDJ
than the ‘coarse’ bagasse pulp, particularly when chemical additives were used.
The possible mechanisms for consolidation of compressible fibrous media are
(from 102):
1. Fibre collapse
2. Bending of fibres & fibre realignment
3. Breaking of fibres
The Australian bagasse fibres are very rigid and significant fibre collapse
was not observed in the microscopy study. Fibre breakage is not occurring due to
the high repeatability of the permeability and compressibility experiments. This
leaves the bending of fibres and fibre realignment.
Further insight is gained by considering the larger improvement in drainage
rate for the ‘depithed’ bagasse when chemicals are added compared to the ‘coarse’
and ‘medium’ pulp samples. It seems unlikely that the chemical additives cause
the Australian bagasse pulp fibres to become more flexible. If there was a
significant bending effect brought about by chemical additives, one would expect
the three different types of pulp (i.e. ‘coarse’, ‘medium’ and ‘depithed’ pulp) to
exhibit a similar change in M and N. This was not observed. Although the ‘30%
depithed’ bagasse pulp had a higher load during steady-state compression when
chemical additives were used, it seems unlikely that it is more rigid than the
‘coarse’ and ‘medium’ pulp samples.
By a process of elimination, it seems the most likely mechanism of
Australian bagasse pulp consolidation is fibre realignment. The use of chemical
additives with bagasse pulp improves fibre realignment by creating a lubricating
effect, binding the “pith” fibres to the larger fibres. When using chemical
additives, the higher the level of pith, the greater the improvement in dynamic
filtration properties (see Figure 4.39).
In the initial stages of pad formation, without chemical additives, the pith
fibres roam freely and block pores in the pulp pad. With chemical additives, the
pith is attached to the longer fibres and are held back in suspension slightly
Chapter 4 - Results and discussion
161
allowing a more porous pad during pad formation. Under the dynamic conditions
in the DDJ, as vacuum increased, the ‘30% depithed’ bagasse pulp pad
consolidated better than the ‘coarse’ and ‘medium’ pulp samples. Under the
dynamic conditions in the compression cell, the ‘coarse’ and ‘medium’ pulp
samples filtered more easily.
For processing low consistency pulp suspensions in conditions similar to a
DDJ, the pulp drains fastest when ‘30% depithed’ bagasse is used in conjunction
with a chemical additives system, but significant vacuum must be applied. The
difficulty with this approach is that the ‘30% depithed’ had a lower water
retention value (255% for ‘coarse’ bagasse pulp and 274% for ‘30% depithed’
bagasse pulp) which makes the sheet harder to dry. Also, the modified DDJ more
closely resembles the Fourdrinier former. It is not known how the filtration of the
pulp samples would compare in a Twin-wire former.
For processing initially networked fibre pads under dynamic conditions,
‘coarse’ bagasse pulp was the most easily dewatered bagasse pulp which
improved when chemical additives are used. The ‘coarse’ bagasse pulp
performed better when either flocculants were not used or when the vacuum level
was low.
4.6.5. Summary of the effect of chemical additives on pulp permeability
and compressibility
It was found that addition of 0.05% CPAM (as Ciba Percol 182) and 0.06%
modified bentonite (as Ciba Hydracol ONZ) improved the retention of bagasse
pulp fines over a wide range of shear using a DDJ.
Applying vacuum to the DDJ had the effect of dramatically reducing the
drainage time. Every bagasse pulp benefitted from the addition of the flocculants
in the DDJ, as measured by reduced drainage time and increased fines retention, at
any level of vacuum.
In steady-state permeability and compressibility experiments, the addition of
flocculants could only be determined to improve Sv for one type of pulp; the ‘30%
Thomas J. Rainey – A study of bagasse pulp filtration
162
depithed’ pulp. However, dynamic filtration experiments showed that there is
also a significant improvement for the ‘medium’ bagasse pulp.
The mechanism of bagasse pulp consolidation is by fibre realignment which
is assisted by chemical additives.
For initially unnetworked suspensions, and the conditions that occur in the
DDJ, the fastest drainage rate was achieved by a standard depithing regime
practiced by industry (i.e. removal of 30% of the shortest fibres) using high levels
of vacuum and chemical additives. These conditions most closely resemble those
of a Fourdrinier former rather than a Twin-wire former. The lower WRV of the
‘coarse’ bagasse pulp indicates that it would dry more quickly.
For initially networked fibre pads, as in the compression cell and
permeability cell, the ‘coarse’ bagasse pulp was the easiest to filter.
Chapter 5 - Conclusions
163
Chapter 5
Conclusions
A study of bagasse pulp was motivated by the possibility of making highly
value-added products from bagasse for the financial benefit of sugarcane millers
and growers. In Australia, there is a perception that bagasse pulp always has poor
filtration characteristics which results in slower paper production compared to
local eucalypt pulp. Surprisingly, there has previously been very little rigorous
investigation into bagasse pulp permeability and compressibility. Only freeness
testing of bagasse pulp has been published in the open literature. Consequently,
this study focussed on improving the filtration properties of bagasse pulp pads.
This study investigated three options for improving the permeability and
compressibility properties of Australian bagasse pulp pads. Firstly, the effect of
the bagasse size, whether ‘coarse’ or ‘medium’ fractions, was considered. The
effect of the mode of juice extraction, whether from a mill or a diffuser, was
determined. Finally the effectiveness of chemical additives, which are known to
improve freeness of pulp slurries, was assessed.
The pre-treated Australian bagasse pulp samples were compared with
samples of eucalypt pulp, depithed Argentinean bagasse pulp that is used
industrially, and a benchmark Australian bagasse pulp that also had 30% of its
shortest fibres removed.
The steady-state permeability and compressibility parameters of bagasse
pulp pads were determined experimentally using two purpose-built experimental
Thomas J. Rainey – A study of bagasse pulp filtration
164
rigs. These parameters were used as inputs for a dynamic filtration model which
more accurately represents industrial paper manufacture. The filtration model was
developed with a view to assist with the development of specialised bagasse pulp
processing equipment. The predicted results of the dynamic model were
compared to experimental data.
The effectiveness of a CPAM and bentonite chemical additives for
improving the retention of fines and increasing the drainage rate of bagasse pulp
slurry was determined in a modified Dynamic Drainage Jar. These chemical
additives were then used to make a pulp pad and their effect on the steady-state
and dynamic permeability and compressibility were determined.
5.1. Findings of this thesis
The most important finding presented in this thesis is that Australian
bagasse pulp was produced with permeability higher than eucalypt pulp, despite a
higher overall fine fibre content. It is hoped that this higher permeability will
enable Australian paper producers to switch from using Australian eucalypt pulp
to bagasse pulp without sacrificing paper machine productivity. The high fibre
stiffness, resulting from thicker fibre walls, and the high proportion of fibres
greater than 1.3 mm in length created a highly permeable bagasse pulp pad. By
fractionating the bagasse and using the ‘flow-through’ reactor appears to have
mitigated the negative influence of the pith particles.
The specific surface area, Sv, for eucalypt pulp was consistent with the
findings of previous workers. The benchmark Australian bagasse pulp had worse
permeability than the eucalypt pulp which is in harmony with the conventional
wisdom which holds that bagasse pulp normally has poor permeability properties.
Australian pulp derived from the ‘coarse’ bagasse fraction had higher
steady-state permeability than the ‘medium’ fraction as measured by the specific
surface area, Sv. However, there was not found to be a difference in bagasse pulp
steady-state permeability between bagasse pulp from a diffuser or a mill.
Chapter 5 - Conclusions
165
The values for the swelling factor, �, were similar for the bagasse pulp
samples and the eucalypt pulp which were all within the ranges reported by
previous workers for wood pulp.
For bagasse pulp, a variable Kozeny factor, k, resulted in a higher value for
� and a lower value for Sv compared with a constant k. This was similar to the
findings obtained for wood pulps reported by Ingmanson (81).
The values for the steady-state compressibility constants M and N were
measured for a wide range of pulp samples. The values for N were generally
consistent with the findings of previous workers for wood pulp, although the
values of M were slightly higher. The bagasse pre-treatment options were not
found to affect the steady-state compressibility parameters of a pulp pad.
The steady-state permeability and compressibility parameters, Sv, �, M and
N, were used in a dynamic filtration model to accurately predict the compressive
load in dynamic filtration of a bagasse pulp pad. The model was particularly
sensitive to Sv, � and N but less sensitive to M.
The dynamic model was particularly accurate for bagasse pulp, provided at
least some pith was removed. The Kozeny-Carman permeability model allowed
the dynamic model to give excellent predictions when a variable Kozeny factor
was used (Equation 2.12), rather than a constant Kozeny factor.
A microparticle chemical additive system, 0.05% CPAM and 0.06%
modified bentonite, improved the retention of bagasse pulp fines over a wide
range of shear using a DDJ. Applying vacuum dramatically reduced the drainage
time. At any level of vacuum, bagasse pulp benefitted from the chemical
additives as measured by reduced drainage time and increased fines retention.
The DDJ was also used to obtain additional information about the behaviour of
thin bagasse pulp mats without flocculants being added.
In steady-state permeability and compressibility experiments involving pulp
pads, the addition of chemical additives could only be determined to improve Sv
for one type of pulp; the ‘30% depithed’ pulp. However, dynamic filtration
Thomas J. Rainey – A study of bagasse pulp filtration
166
experiments showed that there was a small improvement in permeability for the
‘medium’ bagasse pulp.
The mechanism of bagasse pulp consolidation appears to be by fibre
realignment. Chemical additives assist by lubricating the fibres during the
consolidation process.
For initially unnetworked suspensions, and the conditions found in the DDJ
which is similar to Fourdrinier forming, the fastest drainage rate was achieved by
a standard depithing regime practiced by industry (i.e. removal of 30% of the
shortest fibres) using a significant level of vacuum and chemical additives.
However, the lower WRV of the ‘coarse’ bagasse pulp indicates that it would dry
more quickly.
For initially networked fibre pads, as in the compression cell and
permeability cell, the ‘coarse’ bagasse pulp was the easiest to filter.
The physical properties of the ‘coarse’ bagasse pulp were compared to the
benchmark Australian bagasse pulp. The ‘coarse’ bagasse pulp had significantly
improved initial freeness, tear properties and WRV of the pulp. However, the
‘coarse’ bagasse pulp did not have higher tensile strength or burst properties and
had slightly worse apparent density and compressive strength. Also, refining did
not significantly improve any strength property. The bagasse pulp had acceptable
physical properties for the production of generic versions of each paper grade
considered (i.e. photocopier papers, tissues and packaging), by comparison with
Indian bagasse pulp.
In summary, this study has shown that bagasse pulp can be produced with
pulp pad permeability properties that are superior to eucalypt pulp, contrary to
conventional wisdom. The high permeability arises from the stiff pulp fibres and
the high proportion of longer fibres creating an open matrix. Given its higher
pulp pad permeability, ‘coarse’ bagasse pulp could be used for a range of
applications where its properties are superior to conventional bagasse pulp.
Chapter 5 - Conclusions
167
5.2. Recommendations for future work
No optimisation of the cane varieties was performed in the study presented
in this thesis. High fibre energy canes should be developed in Australia and
evaluated for their permeability, compressibility and strength properties.
Increasing the fibre content has the benefits of improving the economy of scale for
a bagasse pulp and paper mill and it also increases the amount of renewable
energy available. Energy canes have the potential to significantly improve the
economics of a bagasse paper industry in Australia. The opinion of the author is
that should a bagasse pulp mill be built in Australia, development of energy canes
would be inevitable.
A dynamic filtration model was developed and verified for bagasse pulp at
ambient conditions. This model will be a valuable tool for assisting the
development of pulp processing equipment that is specially designed for
processing bagasse pulp. This work could be conducted as a further study.
This thesis has outlined methods to improve the filtration properties of
bagasse pulp. Using these methods, the sheet drying performance may now
become the processing step limiting machine production rate. Improving the
sheet drying performance was beyond the scope of this study. Only a few
measurements of WRV were taken. A further study on improving the sheet
drying properties of bagasse pulp would be interesting.
This study recommends careful treatment of bagasse prior to pulping and
the use of chemical additives to improve the filtration properties of bagasse pulp
pads. An investigation into the pulp properties of heavily depithed bagasse pulp
that has been post-processed by pressure screening (as recommended in 50, 64)
may reveal further improvements in bagasse pulp pad permeability.
The issues which are preventing the development of a bagasse pulp industry
in Australia (outlined in section 1.1.3) are (i) the poor filtration properties of
bagasse pulp, (ii) the poor physical strength properties, (iii) high capital cost and
Thomas J. Rainey – A study of bagasse pulp filtration
168
(iv) the remoteness of cane farms to existing pulp mills. For the first issue, there
has recently been significant progress made to improve the filtration properties of
bagasse pulp as outlined in this thesis and also by El-Sharkawy and co-workers
(50, 64). For the second issue, hopefully pulp strength will be improved by
breeding more appropriate cane varieties and changing the juice extraction
method in alignment with the work by Gartside and coworkers (28, 51, 65). For
the third issue, technologies that reduce the capital cost of a bagasse pulp mill
should be explored.
Targeted research into replacing the expensive liquor chemical recovery
plant appears to have great potential for dramatically reducing the capital cost of a
bagasse pulp mill. The ease with which bagasse is pulped makes it a prime
candidate for researching alternative processes which don’t require conventional
chemical recovery technology. Three such alternative processes are: using the
liquor to make fertiliser (50, 64, 145, 146); using electrostatic membranes to
recover pulping chemicals (20); and using organic solvents, such as formic acid,
that can be recovered by distillation (147-150).
References
169
References
(1) Contributors, W., Bagasse, Wikipedia, The Free Encyclopedia,
1 by definition, the reason will be demonstrated shortly
Where h0 is defined as the initial height of the platen above the base and u
0
is the speed of the platen (u0= dh
dt)
recall the governing equation in dimensional form
dφdt
= d
dx ��
��
D(φ) dφdx
−dh
dt
dφdx
we can immediately transform the spatial co-ordinates
dφdt
= d
dX��
��D(φ)
(h0−u
0t)2
dφdX
−u
0
h0−u
0t
dφdX
(C.2.1)
Firstly, we need to establish a couple of simple relations, by the chain rule
dφdx
= δφ
δX
δX
δx+
δφ
δt
δt
δx
but at a constant rate δt
δx=0 so
δφ
δx=
1
h0−u
0t
δφ
δX (C.2.2)
also
x=X(h0−u
0t)→
δx
δt=−u
0X (C.2.3)
By the chain rule, the LHS of (1) becomes
Appendix A-Supplementary material for dynamic filtration modeling
189
δφ
δt=
δφ
δt*δt*
dt+
δφ
δx
δx
δt
substitute (C.2.2) and (C.2.3) and rearranging, the LHS of (C.2.1) reduces to
δφ
δt= u
0
h0
δφ
δt*+ u
0
h0
X
1−t*δφ
δX (C.2.4)
substituting (4) into (3) and rearranging
u0
h0
δφ
δt*= d
dX��
��D(φ)
h2
0(1−t*)2
dφdX
−u
0
h0
1−X
1−t*δφ
δX
Multiplying all terms by (1−t*)2h
0
u0
we get
(1−t*)2 δφ
δt*=
δ
δX ��
��D(φ)
u0h
0
dφdX
−(1−X)(1−t*) δφ
δX
by definition D(φ)= φ(1−φ)K(φ)f'(φ)
µ substituting the definitions of
K(φ)= 1
kS2
v
(1−φ)3
φ2 and f'(φ)=MNφN−1 we get
D(φ)= NM
µkS2
vuoh0
(1−φ)4φN−2
applying the definition of D*(φ) we end up with the non-dimensional
form of the governing equation
(1−t*)2 δφ
δt*=
δ
δX ��
��
D*(φ) dφdX
−(1−X)(1−t*) δφ
δX (C.2.5)
This is the form of the Governing equation required by the FORTRAN
NAB library functions D03PCF and D03PZF
(C.2.5) is subject to the boundary conditions
Boundary condition (1) at X = 1
δφ
δX=0
Boundary condition (2) at X = 0
δφ
δx=
δh
δt
φ
D(φ)
Thomas J. Rainey – A study of bagasse pulp filtration
190
substituting (C.2.2) and the definition of D(φ) we get
1
h0−u
0t
δφ
δX=
u0φ
u0h
0D*(φ)
substituting the definition of t* and rearranging
δφ
δX=
φ(1−t*)
D*(φ)
Appendix A-Supplementary material for dynamic filtration modeling
191
A.3 FORTRAN 77 program for the dynamic filtration model
**************************************************************** * * Tom Raineys pulp compression modelling program * adapted from * D03PCF Example Program Text * * VERSION 6: FINAL * * Assisted by Neil Kelson * * This version is used for bagasse pulp compression modelling * by adjusting the parameters below * * * To compare with experimental data need to specify PhiInit; Hinit; MPHI; * NPHI; Sv; and DHDT. * Generates two files: fort.21 (output for Ps) and fort.22 (output for phi) * **************************************************************** * .. Parameters ..
IND = 0 ITASK = 1 * * Set spatial mesh points * PIBY2 = 0.5D0*X01AAF(PI) HX = PIBY2/(NPTS-1) X(1) = 0.0D0 X(NPTS) = 1.0D+0 DO 20 I = 2, NPTS - 1 X(I) = SIN(HX*(I-1))
20 CONTINUE * * Set initial conditions * TS = 0.0D0 TOUT = 0.1D-5 * for testing - reduce TOUT step
Appendix A-Supplementary material for dynamic filtration modeling
193
* TOUT = 0.05D0 * Tom:
WRITE (NOUT,99999) ACC, PHIINIT, MPHI, NPHI WRITE (NOUT,99998) (XOUT(I),I=1,50) * Tom: Change from I=1,6 to I=1,50 to accommodate new X columns * Set the initial values CALL UINIT(U,NPTS)
ILOOPS = 87 * Tom: Changed above line from 5 to 83 to get values of TOUT 0 to * 0.83(=75mm/90mm at constant rate)
DO 40 IT = 1, ILOOPS
IFAIL = -1 TOUT = 0.01D0+TOUT * Tom: Introduce a linear timestep * * Call the solver CALL D03PCF(NPDE,M,TS,TOUT,PDEDEF,BNDARY,U,NPTS,X,ACC,W,NW,IW, + NIW,ITASK,ITRACE,IND,IFAIL) * * Interpolate solution at required spatial points CALL D03PZF(NPDE,M,U,NPTS,X,XOUT,INTPTS,ITYPE,UOUT,IFAIL)
WRITE (NOUT+1,99996) TOUT, (UOUT(1,I,1),I=1,INTPTS) WRITE (NOUT,99995) TOUT, ((MPHI*(UOUT(1,I,1))**NPHI),I=1,INTPTS) * Tom: to alternate between PS output and PHI output make active the correct * line here and also make active the appropriate FORMAT line (i.e. * either 99996 or 99995).
Thomas J. Rainey – A study of bagasse pulp filtration
194
+ ' MPHI = ',D12.5,/ + ' NPHI = ',D12.5,/) 99998 FORMAT (' T/ X ',50F8.4,/) 99997 FORMAT (' Number of integration steps in time ', + I4,/' Number of residual evaluations of resulting ODE ' + 'sys', + 'tem',I4,/' Number of Jacobian evaluations ', + ' ',I4,/' Number of iterations of nonlinear solve', + 'r ',I4,/) 99996 FORMAT (1X,F8.4,' PHI',50F8.4) * Tom: Change above format from 6F8.4 to 50F8.4 to display new columns * Tom: for X in output as governed by INTPTS 99995 FORMAT (1X,F8.4,' PS',50F8.3) * Switch between 99996 and 99995 END **************************************************************************** SUBROUTINE UINIT(U,NPTS) * Routine for PDE initial conditon
Appendix A-Supplementary material for dynamic filtration modeling
195
COMMON /VBLE/PHIINIT,MPHI,NPHI
* NPHI (-) and MPHI (Pa) are the exponent and pre-exponent for the * compression correlation * mu is the viscosity (Pa.s) * koz is the variable kozeny factor (or set to 5.55) * Sv is the specific surface area (m^-1) * DHDT is the rate of the piston (m/s) * HINITIAL is the initial height (m) * Average Milled bagasse pulp MPHI * Average Milled bagasse pulp NPHI
MU = 0.001D0 * Viscosity of water in Pa.s KOZ = 3.5*((1-3.5*U(1))**3)*(1+(57*((3.5*U(1))**3)))/ + ((3.5*U(1))**0.5) * Koz can be set as constant k=5.55 if desired SV = 191700D0 * SV as m-1; this value is for optimum for variable koz factor
DHDT = 4.1667D-4 * in m/s - 75 mm displacement over 3 MINS HINITIAL = 0.073D0 * Initial height of the platen is 90mm, height in metres